A wheel of an automobile and a rotating member in a brake system are rotatably supported to a suspension device via a wheel-supporting bearing unit. FIG. 4 shows an example of a conventionally known wheel-supporting bearing unit. This wheel-supporting bearing unit includes a hub 1, an outer ring 2, and a plurality of balls 3 which are rolling elements. The hub 1 is formed by combining together a hub body 4 and an inner ring 5.
The hub body 4 has a mounting flange 6 disposed on a portion near to an outer end of an outer peripheral surface of the hub body 4 and adapted to support the automotive wheel and the rotating member for brake system; a first inner ring raceway 7a disposed on an intermediate portion of the hub body 4; and a small-diameter stepped portion 8 disposed on an inner end portion of the hub body 4 and having a smaller outside diameter dimension than this first inner ring raceway 7a. (Here, the outer side with respect to the axial direction refers to the widthwise outer side of an automobile when the wheel-supporting bearing unit is incorporated into the automobile, i.e., the left side in FIGS. 4 to 6. Conversely, the right side in FIGS. 4 to 6, which is the widthwise central side of the automobile, is referred to as the inner side with respect to the axial direction. This is true throughout the specification.) The inner ring 5 is fitted over this small-diameter stepped portion 8, and the inner ring 5 is fixed to the hub body 4 by a caulked portion 9 formed by plastically deforming an inner end portion of the hub body 4 in a radially outward direction. A second inner ring raceway 7b is formed in an outer peripheral surface of this inner ring 5.
The outer ring 2 has formed in its inner peripheral surface a first outer ring raceway 10a opposing to the first inner ring raceway 7a and a second outer ring raceway 10b opposing to the second inner ring raceway 7b, and has a coupling flange 11 formed on its outer peripheral surface for coupling and fixing to the suspension device. The plurality of halls 3 are rotatably provided between each of the both first and second inner ring raceways 7a and 7b and each of the both first and second outer ring raceways 10a and 10b while retaining by an unillustrated retainer. By this configuration, a double-row angular contact ball bearing in a back-to-back combination is formed to support the hub 1 on the inner side of the outer ring 2 rotatably and in such a manner as to be capable of bearing the radial load and the thrust load. It should be noted that although in the illustrated example, the balls 3 are used as rolling elements, tapered rollers can be used when a bearing unit is used for an automobile having heavy weight.
In addition, in the illustrated example, as diameters DA1 and DA2 of the both first and second inner ring raceways 7a and 7b are set to be mutually equal (DA1=DA2), and diameters DB1 and DB2 of the both first and second outer ring raceways 10a and 10b are set to be mutually equal (DB1=DB2), a pitch circle diameter PCD1 of each of the balls 3 (first rolling element row) provided between the first inner ring raceway 7a and the first outer ring raceway 10a and a pitch circle diameter PCD2 of each of the balls 3 (second rolling element row) provided between the second inner ring raceway 7b and the second outer ring raceway 10b are set to be mutually equal (PCD1=PCD2).
When the wheel-supporting bearing unit constructed as described above is used, the coupling flange 11 is coupled and fixed to the suspension device, and the wheel and the rotating member for brake system are supported by and fixed to the mounting flange 6. As a result, the wheel and the rotating member for brake system can be rotatably supported to the suspension device.
Incidentally, mainly during the turning of the automobile, a moment load based on a road surface reaction force is applied to the above-described wheel-supporting bearing unit. At that time, as for the wheel-supporting bearing unit, the hub 1 undergoes elastic deformation so as to be bent with respect to the outer ring 2, and a center axis of the mounting flange 6 tends to be inclined with respect to a center axis of the coupling flange 11. The inclination thus produced exerts an adverse effect on the traveling stability and braking performance of the automobile. For this reason, in view of improving these performances, as the wheel-supporting bearing unit, it is desirable to adopt a structure capable of sufficiently suppressing the above-described inclination, i.e., a structure capable of sufficiently enhancing the moment rigidity.
Incidentally, the greater the distance L between points of application P1 and P2 of the both rolling element rows (points of intersection between the respective lines of action, α1 and α2, of the both first and second rolling element rows and a center line β of the wheel-supporting bearing unit), the greater the moment rigidity of the wheel-supporting bearing unit. Meanwhile, the positions of the both points of application P1 and P2 on the center line β move in the direction of moving away from each other as pitch circle diameters PCD1 and PCD2 of the both rolling element rows become large. Accordingly, the distance L between points of application P1 and P2 can be set to be large by increasing the pitch circle diameters PCD1 and PCD2 of the both rolling element rows. However, if both of these pitch circle diameters PCD1 and PCD2 are increased, the wheel-supporting bearing unit becomes unnecessarily large and the weight increases. This result becomes contrary to the demand for compact size and lightweight, and is therefore undesirable.
In contrast, patent document 1 describes a wheel-supporting bearing unit shown in FIG. 5. In comparison with the structure shown in FIG. 4, in the structure shown in FIG. 5, only the pitch circle diameter PCD1 of the first rolling element row is set to be large (PCD1>PCD2) by increasing the diameters DA1 and DB1 of the first inner and outer ring raceways 7a and 10a without changing the pitch circle diameter PCD2 of the second rolling element row. As a result, the point of application P1A of the first rolling element row is moved away from the point of application P2 of the second rolling element row so as to enlarge the distance LA between these both points of application P1A and P2 (LA>L), thereby enhancing the moment rigidity of the wheel-supporting bearing unit. In the wheel-supporting bearing unit thus constructed, since only the PCD1 is set to be large to enhance the moment rigidity, it is possible to prevent the wheel-supporting bearing unit from becoming unnecessarily large and its weight from increasing.
In addition, the above-described patent document 1 also describes a wheel-supporting bearing unit shown in FIG. 6. In comparison with the structure shown in FIG. 5, in the structure shown in FIG. 6, only the diameter DA1 of a first inner ring raceway 7c is made large and the diameter of each ball 3a constituting the first rolling element row is made small without changing the diameter DB1 of a first outer ring raceway 10c. As a result, by making the pitch circle diameter PCD1 of this first rolling element row even larger to move the point of application P1B of this first rolling element row further away from the point of application P2 of the second rolling element row, thereby further enlarging the distance LB between these both points of application P1B and P2 (LB>LA). By adopting such a construction, the moment rigidity of the wheel-supporting bearing unit is further enhanced.
It should be noted that, in the structure shown in FIG. 6, the total number of these balls 3a is increased by the portion by which the diameter of each ball 3a constituting the first rolling element row is made small, to thereby make it possible to disperse the load applied to these balls 3a. In addition, in the structures shown in FIGS. 4 and 5, the radii of curvature of cross-section profiles of the first and second outer ring raceways 10a and 10b are respectively R and are equal. However, in the structure shown in FIG. 6, as the diameter of each ball 3a constituting the first rolling element row is made small, as described above, the radius of curvature “r” of the cross-section profile of the first outer ring raceway 10c is smaller than the radius of curvature “R” of the cross-section profile of the second outer ring raceway 10b (r<R). In any cases, in the wheel-supporting bearing unit thus constructed, as compared with the structure shown in FIG. 5, since the diameter of the first outer ring raceway 10c is not changed, it is possible to maintain the same outside diameter dimension as that of the structure shown in FIG. 5 while enhancing the moment rigidity.
When the outer rings 2, 2a and 2b constituting the various wheel-supporting bearing units described above are manufactured, the both first and second outer ring raceways 10a (10c) and 10b are respectively subjected to a grinding process as a finishing process. As a specific grinding method, patent document 2 describes a method in which the grinding process is sequentially carried out with respect to the both first and second outer ring raceways 10a (10c) and 10b one by one. However, if such a method is carried out, the operational efficiency deteriorates since the Labor of providing the grinding process for the both outer ring raceways 10a (10c) and 10b becomes cumbersome and complicated.
On the other hand, the aforementioned patent document 2 describes a method which is aimed at the structure shown in FIG. 4 {the structure in which the diameters DB1 and DB2 of the first outer ring raceway 10a and the second outer ring raceway 10b are mutually equal (DB1=DB2)} and in which the both outer ring raceways 10a and 10b are simultaneously subjected to the grinding process by one grinding tool having a grinding wheel portion for grinding the first outer ring raceway 10a and a grinding wheel portion for grinding the second outer ring raceway 10b. According to such a method, it is possible to improve the operational efficiency at the time of subjecting the both outer ring raceways 10a and 10b to the grinding process and attain a reduction in the cost of the fabrication apparatus. However, the above-described method is aimed at the structure shown in FIG. 4 (the structure in which DB1=DB2) and is not aimed at the structures shown in FIGS. 5 and 6 (the structures in which DB1>DB2). In other words, the aforementioned patent document 2 does not describe a specific operational procedure which is aimed at the structures shown in FIGS. 5 and 6 (the structures in which DB1>DB2) and when the both first and second outer ring raceways 10a (10c) and 10b are simultaneously subjected to the grinding process.
In addition, when the outer rings 2, 2a and 2b constituting the various wheel-supporting bearing units described above are manufactured, in general, the both first and second outer ring raceways 10a (10c) and 10b are respectively subjected to a high-frequency quenching process as a hardening process. In the case where such a high-frequency quenching process is provided, the both first and second outer ring raceways 10a (10C) and 10b are subjected to high-frequency heating. As a specific heating method, patent document 3 describes a method which is aimed at the structure shown in FIG. 4 (the structure in which DB1=DB2) and in which the both outer ring raceways 10a and 10b are simultaneously subjected to high-frequency heating by a high-frequency heating coil having a coil portion for heating the first outer ring raceway 10a and a coil portion for heating the second outer ring raceway 10b. However, this method is aimed at the structure shown in FIG. 4 (the structure in which DB1=DB2) and is not aimed at the structures shown in FIGS. 5 and 6 (the structures in which DB1>DB2). In other words, the aforementioned patent document 3 does not describe a specific operational procedure which is aimed at the structures shown in FIGS. 5 and 6 (the structures in which DB1>DB2) and when the both first and second outer ring raceways 10a (10c) and 10b are simultaneously subjected to high-frequency heating.    [Patent Document 1] Japanese Patent Unexamined Publication JP-A-2004-108449    [Patent Document 2] Japanese Patent Unexamined Publication JP-A-2004-92830    [Patent Document 3] Japanese Patent Unexamined Publication JP-A-59-226118